There has been known an auto optical focus detecting device of a camera having an auto focus optical system. FIG. 1 is a schematic view showing an optical system of an auto optical focus detecting device of, for example, a single-lens reflex camera which has the auto focus optical system. In the figure, 1 denotes a photographic lens, 2 a subject to be photographed, 3 a view field mask, 4 a condenser lens, 5 a diaphragm mask, 6 and 7 a separator lens as an image splitting optical element, and 8 a CCD as a image receiving element. The view field mask 3, the condenser lens 4, the diaphragm mask 5, the separator lenses 6 and 7, and the CCD 8 are integrally modulated as one unit and constitute an auto focus optical system 9.
In this auto focus optical system 9, the view field mask 3 is disposed in the vicinity of a film equivalent plane 10. The film equivalent plane 10 is in a position optically conjugate with the subject 2 to be photographed through the photographic lens 1. A well focussed image 11 of the subject 2 is formed on the film equivalent plane 10 when the photographic lens 1 is in focus. The condenser lens 4 and the diaphragm mask 5 have the function of splitting the photographic light passing on both right and left sides of the photographic lens 1. The separator lenses 6 and 7 are in a position optically conjugate with the photographic lens 1 through a condenser lens 4.
The separator lenses 6 and 7, as shown in FIG. 2, are disposed in the horizontal direction. Further, the separator lenses 6 and 7 face imaginary opening areas 14 and 15 of an exit pupil 13 of the photographic lens 1 through a zone 12 located in a position optically conjugate with a center zone of a finder as will be described. The separator lenses 6 and 7 intake a bundle of rays passed through the opening areas 14 and 15. The image 11 formed on the film equivalent plane 10 is reimaged as images 11' in two areas on the CCD 8.
A distance between the images 11' well focussed (see FIG. 3(a)) is represented by l.sub.o as shown in FIG. 4. When the photographic lens 1 is focussed in a position in front of shown in FIG. 3(b), the distance between the images 11' becomes less and, as a result, the distance between signals S corresponding thereto becomes less than the distance l.sub.o. On the contrary, when the photographic lens 1 is focussed in a position behind, the focal point of the afore-mentioned well focussed image as shown in FIG. 3(c), the distance between the images 11' becomes greater and, as a result, a distance between signals S corresponding thereto becomes greater than the distance l.sub.o. Since the distance between the images 11' is changed in proportion to a defocussing amount of the photographic lens 1, in the conventional auto optical focus detecting device of a single-lens reflex camera, a distance between images of the CCD 8 is detected and the signals are arithmetically processed, and the photographic lens 1 is moved to the focal position with reference to the focussing direction and defocussing amount of the photographic lens 1. And, as shown for example in FIG. 5, if the optical focus is found by framing as such that a desired subject 2 to be photographed is located in the center zone 17 arranged at the center of the finder 16, the photographic lens 1 is automatically brought to a focussing state. If a photograph is taken in the foregoing state, a well focussed photograph can be obtained.
In this conventional auto optical focus detecting device of a single-lens reflex camera, since the zone is located in the center of the finder 16, a desired subject 2 will be positioned in the center of an obtained photograph unless an adequate arrangement is made. However, there are some instances where a desired subject 2 is preferably positioned in the peripheral area of a photograph instead of the center thereof. To this end, thereofore, in the conventional single-lens reflex camera, a focus lock mechanism is provided. That is, the subject 2 to be photographed is positioned in the center of the finder 16 to automatically find the distance to the subject 2. In that state, the focus is locked. If a photograph is taken in the framing as shown in FIG. 6, a photograph can be obtained in which a desired subject 2 is positioned in the peripheral area.
However, in this conventional auto optical focus detecting device of a single-lens reflex camera, the subject 2 must first be positioned in the center of the finder 16. Then, the photographic lens 1 must be moved to a focussing state. In that state, the focus must be locked to fix the photographic lens 1. Then, the framing must be performed once again. Only thereafter, a photograph can be taken. Therefore, much time and labor are required before the camera is ready to take a photograph.